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Most important, the study's primary efficacy endpoint will measure the ability of GBT440 to increase levels of oxygen-carrying hemoglobin in sickle cell patients compared to a placebo, Global Blood said.

That's a potentially easier efficacy threshold for GBT440 to achieve in sickle cell disease compared to what's historically been required by the U.S. Food and Drug Administration.

Typically, FDA has mandated any drug aimed at treating sickle cell disease demonstrate a reduction in the frequency of painful and damaging sickle cell crises. These "vaso-occlusive crises" are caused by clumps of sickled red blood cells blocking blood vessels.

Global Blood says the FDA agreed on the design of the GBT440 phase III study, including the use of the hemoglobin primary endpoint. However, the agreement on study design and the data required for the drug's approval is not guaranteed in writing with a Special Protocol Assessment (SPA.)

"We did not pursue an SPA. Had we done it, we could have gotten one but it would have taken longer," said Global Blood CEO Ted Love, in a phone interview Monday afternoon.

Global Blood wants to start screening sickle cell disease patients for the phase III study in December with the hope of having top-line results in the first half of 2019.

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